Heat exchanger systems which vary the path of fluid flowing therethrough in response to a change in the characteristics (e.g. temperature, pressure, etc.) of the fluid are old in the art. For example, WO 94/29659 shows a plate-type oil cooler which has a pressure-responsive valve assembly connected to the inlet of the oil-side to permit the oil to bypass the oil-side of the cooler when the pressure on the oil-side of the cooler exceeds a predetermined value. Alternatively, U.S. Pat. No. 4,669,532 discloses a bimetallic valve which is disposed in the oil-side of an oil cooler to permit the oil to bypass the oil-side of the cooler when the temperature of the oil is below a predetermined value.
Additionally, there are numerous examples of heat exchanger systems wherein the flow rate of a fluid flowing through a heat exchanger is controlled according to the temperature of that of another fluid flowing through the heat exchanger. For example, German Laid-Open Application No. 196 37 818 and European Laid-Open Application No. 787 929 show two such systems wherein the flow of coolant through an oil cooler is controlled in response to the temperature of the oil flowing through the heat exchanger. In both of the systems, a thermostat is located upstream of the inlet to measure the oil temperature before the oil enters the heat exchanger, although it is also known to control the flow of coolant through the heat exchanger system in response to the oil temperature as it exits the heat exchanger.
The problem with these systems is that they may take up considerable amounts of space, which is always at a premium in automotive applications, a primary use of this art. Additionally, these systems may add weight to the vehicle to which they are attached, possibly degrading fuel economy thereby. Furthermore, the environment surrounding the thermostat in these systems may affect the oil temperature reading, causing more or less coolant to be directed to the heat exchanger than is actually necessary.